Skip to main content
Open Access Publications from the University of California


The International Journal of Comparative Psychology is sponsored by the International Society for Comparative Psychology. It is a peer-reviewed open-access digital journal that publishes studies on the evolution and development of behavior in all animal species. It accepts research articles and reviews, letters and audiovisual submissions.

Volume 32, 2019

Research Article

Domestication and the role of social play on the development of socio-cognitive skills in rats

Several studies on rats and hamsters, across multiple laboratories, have shown that limiting play in the juvenile period leads to adults that have physiological and anatomical changes in the medial prefrontal cortex (mPFC) and reduced socio-cognitive skills. Peers raised with playful peers have better socio-cognitive skills than animals raised with adult partners. Using Long Evans hooded rats - a commonly used domesticated strain - this relationship has been replicated multiple times. However, when the same paradigm was used with laboratory-reared wild rats, no differences were found between rats reared with peers and ones reared with adults. It has been shown that the key play-generated experiences involved are those related to actively wrestling with a partner and turn taking (as measured by role reversals), which give both partners opportunity to gain the advantage during play fighting. In the present study, we tested the hypothesis that wild rat adults provide juveniles more such experiences than do adult Long Evans rats. The asymmetry in the play interactions in adult-juveniles pairs was compared between the two strains. As predicted, wild rat adults initiated more play with the juveniles, wrestled more and provided more opportunities for role reversals. The findings thus support the hypotheses for the observed strain differences in the effects of rearing condition on the mPFC.

The role of outcome unit size in the collective foraging strategies of rats

The distribution of foraging strategies and associated activities of Wistar rats was examined, with food outcomes presented in small vs. large units. Groups of 4 rats foraged for food in a 4 x 3 array of covered holes, some containing 4 g of food. For some groups, food consisted of shelled sunflower seeds (small units); for other groups, food consisted of 3 pellets of chow (large units). Foraging strategies were classified as either production (seeking patches with food) or scrounging (tracking conspecifics). Production strategies were more common among groups that foraged for pellets instead of seeds. Producing food was highly correlated with contacting gates covering holes, whereas scrounging for food was highly correlated with following others in the group. The prevalence of activities associated with each foraging strategy was highly correlated with the proportion of time spent consuming food obtained from each activity (i.e., produced vs. scrounged food). Taken together, these findings suggest that, similar to other species, the finder’s advantage (low with small units, high with large units) modulates social foraging strategies in rats. A simple outcome-strategy feedback mechanism appears to mediate this modulation.

  • 4 supplemental images

Response-inhibition during problem solving in sheep

Response inhibition is a behavioural skill that is important for flexible behaviour and appropriate decision making. It requires the suppression of a prepotent, but inappropriate action, in order to achieve a more advantageous outcome. Response inhibition has been tested in many animal species using the cylinder task. This task requires the self-driven inhibition of an impulse to obtain a visible food reward via a detour, rather than a direct but blocked route. We have shown previously using the stop-signal task that sheep can successfully interrupt an already-started response, if a reward is going to be restricted. However, it is not known if sheep can show self-driven response inhibition in a task that provides a reward independent of performance. Here we tested two groups of sheep on the cylinder task (11 Lleyn sheep: aged 8 months; 8 Welsh mountain sheep aged ~8 years old). Sheep were trained using an opaque cylinder and all sheep successfully learned the task. When response inhibition was tested using the transparent cylinder, all sheep performed significantly better than chance, but the older sheep showed a reduced number of correct responses compared to the young sheep (72.5±5.0% and 86.4±4.3% respectively). The results show that sheep have a mechanism for self-regulating their actions in order to retrieve food faster.


  • 3 supplemental files

Three Levels of Consciousness: A Pattern in Phylogeny and Human Ontogeny

Investigations in the cognitive abilities of different animal species and children at different ages have revealed that consciousness comes in degrees. In this review, I will first address four cognitive abilities that are important to discriminate levels of consciousness: mirror self-recognition, theory of mind, mental time travel, and the capacity to entertain secondary representations. I will then examine putative relations between these abilities and assign them to three levels of consciousness (anoetic, noetic, autonoetic). Finally, I will discuss the implications of differences in consciousness for the understanding of behavioral organization in animals and humans and for animal welfare science. I will argue that, on one hand, implicit behavioral rules may account for results obtained in research on theory of mind and mental time travel abilities in animals and children. On the other hand, secondary representations may be the key to explain behaviors based on semantic memory as well as semantic future planning abilities observed in great apes and young children. These considerations are in accordance with the view that an explicit theory of mind and a continuous self through time are unique to humans.

Brief Report

A Facet Theory Analysis of the Structure of Cognitive Performance in New Zealand Robins (Petroica longipes)

In this report we analyse the cognitive performance of New Zealand Robins (Petroica longipes) using facet theory, smallest space analysis (SSA) and partial order scalogram analysis (POSA). The data set we analyse was originally subjected to principle component analysis in order to develop a test-battery for avian cognitive performance. We extend these analyses by proposing a two facet rather than a single component solution using SSA and we characterize individual birds by their scores on all tasks using POSA. We note problems with the small sample size and call for our exploratory analyses to be replicated using a larger sample of birds and for the development of further test items using the facet theory’s tool the mapping sentence. We suggest that facet theory and the mapping sentences are research approaches suitable for conceiving, designing, analysing and developing theory that may be used within avian cognitive research. We conclude by proposing a mapping sentence for avian cognition, which forms an adaptable template for future avian cognition research.

Special Issue on Contact

Methodological Considerations for Comparison of Cross-species Use of Tactile Contact

Cross-species comparisons are benefited by compatible datasets; conclusions related to phylogenetic comparisons, questions on convergent and divergent evolution, or homologs versus analogs can only be made when the behaviors being measured are comparable. A direct comparison of the social function of physical contact across two disparate taxa is possible only if data collection and analyses methodologies are analogous. We identify and discuss the parameters, assumptions and measurement schemes applicable to multiple taxa and species that facilitate cross-species comparisons. To illustrate our proposed guidelines for evaluating the role played by tactile contact in social behavior across disparate taxa, this paper presents data on mother-offspring relationships in the two species studied by the authors: chimpanzees (Pan troglodytes schweinfurthii) and dolphins (bottlenose and spotted, Tursiops truncatus and Stenella frontalis, respectively). Cross-species comparative studies allow for a more comprehensive assessment of the similarities and differences with respect to how animals traverse the relationships that form their social groups and societies.